The Sun emits hazardous radiation in the form of SPEs (Solar Particle Events). These charged and highly energetic particles can interact with materials and cause: lattice deformation via ionization (both directly and indirectly); lattice deformation by direct collision; impurity creation in the lattice by neutron capture (neutrons produced in primary interactions); energy deposition which can lead to build up of charge, thus distorting the lattice. In addition to SPEs, there are also Galactic Cosmic Rays (GCR), energetic charged particles from Deep Space. These are less apparent because the solar activity deflects the GCR. Occasionally, the Sun emits a coronal mass ejection (CME) which is seen alongside a solar flare. CMEs release a much greater flux of SPEs and have been known to cause mission failure in satellites or spacecraft.
On Earth, the magnetosphere provides protection from the harmful radiation from the Sun and Deep Space by deflecting the majority of high energy particles. In addition, the atmosphere attenuates the energetic particles that do pass through the magnetosphere. However, when in orbit around the Earth, this protection is diminished due to lack of atmosphere and weakening of the magnetosphere with distance from the Earth's core. It is particularly harmful to have the satellites orbiting within the radii of the Van Allen belts due to their nature as regions of trapped electrons and protons within the inner magnetosphere, between 60 and 6000 miles altitude for the inner belt and between 8,400 to 36,000 miles for the outer belt. These radii vary with solar activity, so specific orbital tracks must be considered. Different elevation orbits have different associated dose rates, resulting in different total doses depending on the mission lifetime. A higher total dose is more harmful to both humans and equipment. It is also believed that a higher dose rate can have a more detrimental effect. In a geostationary orbit for 20 years, the expected total dose is 137 Gy from behind 10 mm thick aluminum shielding. In low earth orbit (LEO) the dose is less than half this value.
Both SPE and GCR lead to distortions in the lattice of a material and have specific effects on different types of materials. In optical materials, this can lead to defect creation via the previously mentioned processes in the fiber lattice structure. Not only is there creation of new defects, but intrinsic defects are also exacerbated. The radiation induced defects will absorb light, so that less light can propagate the fiber, resulting in so-called radiation induced attenuation (RIA). In order to keep the fiber as radiation tolerant as possible, it is therefore important to keep as perfect a lattice structure as possible.
It has previously been reported that a full recovery from radiation damage to optical fibers can be found from pumping at both 980 nm and 532 nm. It is, however, not ideal in space to have multiple pumps since this requires more mass, power and adds another layer of complexity in reliability testing.
It will be appreciated that the opportunities to repair or replace optical fibers located on satellites or spacecraft are very limited. Even if repair or replacement is possible, it is typically a very expensive process.
Furthermore, given the use typical applications of optical fibers within satellites and/or spacecraft, any damage thereto could have very serious implications. For example, the optical fibers may be utilized within a spacecraft's navigation system, and hence any damage thereto may cause the spacecraft to take an incorrect path.
It is therefore essential to ensure that optical fibers used on satellites and spacecraft are optimally resistant to damage, particularly radiation damage.